Can you return multiple values in a C++ function?

Question

I have to use recursion to figure out how many 2's and 5's it will take to make an integer greater than 4. For example with the integer 14, you need two 2's and two 5's. For 13, you need four 2's and one 5.

It basically works like this. As you count up, if there is more than two 2's then you take away two 2's and add a 5, otherwise you minus one away from the 5's and add three 2's. I am asked to do this in a recursive function but i am unsure as to how. The inputs are the integer wanted and I assume the 2's and 5's. So i think it is

fcnName(n,two,five)
{
if(n==4)
return two 2's and zero 5's

unsure of what else to put in here
}

Answer 1

Best Answer - Chosen by Asker

you can only return 1 variable using "return". there is a way to do what you want though. basically you pass the address of the "two" and "five" variables.
example:

int n, two, five;
fcnName(n, &two, &five) //no need to return anything
{
//your function code
}

Asker's Rating:

Asker's Comment:

thanks

Answer 2

The easy way to pass multiple values around is to create a single object with the variables inside, then pass that one object around. This can be done with a class or a struct. Here's an incomplete example...

class ResultsObject {
int subtotal = 0;
int twos = 0;
int fives = 0;
}

// the recursive function

ResultsObject doSomething( int target, ResultsObject results ) {
if (we_are_done) {
return results;
}

if (we_should_add_five) {
results.fives += 1;
results.subtotal += 5;
}
else {
results.twos += 1;
results.subtotal += 2;
}

return doSomething(target, results);

}

// main

void main ()
{
int target = 14;
ResultsObject scratch, answer;

answer = doSomething (target, scratch);
}


That's one way to do it, probably close to what you were thinking of. You could also do it by leaving the variables in one place and passing a pointer around (you don't need to return the pointer); or you could actually make the recursive function a member of the class itself so you have instant access to the variables and only have to send the answer back. I'll leave that as an exercise for the student...

Answer 3

#include <iostream>
#include <fstream>
#include <string>
using namespace std;


string twoFive(int n,int &two, int &five){
string note;
if((n<=0)||(n==1)){
note="";
cout<<"the number have "<<two<<" twos and "<<five<<" fives"<<endl;
return note;
}else {
if((n-2>0)&&(two<2)){
two++;
n-=2;
note = twoFive(n,two,five);
}else if((n-5>=0)&&(five<=2)){
five++;
n-=5;
note = twoFive(n,two,five);
}
return note;
}
}

int main(){
int tr=13;
int two=0;
int five=0;
cout<<twoFive(tr,two,five);
cout<<"the number "<<tr<<" have "<<two<<" twos and "<<five<<" fives"<<endl;
return 0;
}

this is your program but think how it works.

Answer 4

Dear,

An Alternative Syntax for Multiple Return Values in C-based Languages

Most functions do not need more than one return value, but when you need
more there's no easy way around it. Java has this problem, functions are limited to one return value, and in my experience it is often complicated
to get along with this limitation. In most cases you either write more
functions than necessary or you need to create a additional classes
that need even more constructors, accessor methods and documentation.


C++ and C# allow multiple return values using pointers and references. This solves the problem,
but I think it feels wrong and especially the C++ syntax does not
create very readable code. So I am going to show you an alternative, derived
from Python's tuples. But first the existing syntax variants and their
disadvantages:



Classic C syntax

The classic C syntax for multiple return values is to use a pointer.
Here is an example function that parses an integer in a string. It returns
a boolean to show whether it was parsed successfully and the integer itself:

// implementation:
int parseInt(const char *str, bool *success) {
const char *s = str;
int r = 0;
while (*s) {
char c = *s;

if ((c < '0') || (c > '9')) {
*success = false;
return 0;
}
r = r * 10 + (c - '0');
}
*success = true;
return r;
}

// invocation:
bool s;
int v = parseInt("2004", &s);


Disadvantages:


Neither declaration nor invocation syntax indicate whether 'success' is really
a return value. It may also be just an optimization for an input value
(admittedly unlikely in this example) or may be both input and output value. Only the documentation and the implementation can help

You can not find out whether null is allowed for 'success' without
looking at the documentation or the implementation

The compiler won't catch a bug if 'success' is not initialized before
returning in some code paths, because it does not know the purpose of 'success'



Classic C syntax with null

This is the same as above, but it allows a 0 for 'success' in order to
make it optional:

// implementation:
int parseInt(const char *str, bool *success = 0) {
const char *s = str;
int r = 0;
while (*s) {
char c = *s;
if ((c < '0') || (c > '9')) {
if (success)
*success = false;
return 0;
}
r = r * 10 + (c - '0');
}
if (success)
*success = true;
return r;
}

// invocation
int v = parseInt("2004");


Disadvantages:

You still need to look at documentation/implementation to find out what
success is good for

The compiler will still not notice when success has not been set
before returning, and the check whether 'success' is null adds another
potential error

Two additional lines-of-code were needed in the implementation
to make success optional


C++ syntax with references


// implementation:
int parseInt(const char *str, bool &success) {
const char *s = str;
int r = 0;
while (*s) {
char c = *s;
if ((c < '0') || (c > '9')) {
success = false;
return 0;
}
r = r * 10 + (c - '0');
}
success = true;
return r;
}

// invocation:
bool s;
int v = parseInt("2004", s);


Advantages:

References do not have the 'null' issue or other pointer problems


Disadvantages:

The invocation does not have any hint that the second argument will be
modified. This can make code very hard to read if you do not know
the functions, because any function may modify any argument

You still do not know whether 'success' is a input or an output
value

Default values are not possible, you always need to have a bool even
if you do not look at it

The compiler won't notice the bug when 'success' is not initialized in
some code paths, because it does not know the purpose of 'success'



C# syntax

This is the same function in C#. IMHO the C# syntax is vastly superior to
the C++ alternatives:

// implementation:
int parseInt(String str, out bool success) {
char s[] = str.ToCharArray();
int r = 0;
foreach (char c in s) {
if ((c < '0') || (c > '9')) {
success = false;
return 0;
}
r = r * 10 + (c - '0');
}
success = true;
return r;
}

// invocation:
bool s;
int v = parseInt("2004", out s);


Advantages:

It's obvious in declaration and invocation that 'success' is an output
argument (in/out arguments use the keyword 'ref')

The compiler can check whether 'success' has been set by the function
before returning

There are no pointer issues
Disadvantages:

Default arguments are not possible (a C# limitation)
You always need to declare the bool before invoking the function




Using Python-style tuples


An alternative to the C# syntax would be using Python-like tuples.
Tuples are comma-separated values in parentheses that can be on the left and right
side of an assignment statement. The syntax would look like this:

int x, y, z;
(x, y, z) = (1, 2, 3);

// The equivalent of the last line is:
x = 1;
y = 2;
z = 3;

// The source tuple can contain expressions as items:
(x, y) = (z-2, 5*5);

// the right side can have more items than the left (but not the other way round):
(x, y) = (1, 2, 3, 4, 5);

// the left side can be a regular value; then only the first item is taken:
x = (1, 2, 3);

// local variable declaration in a tuple
(int a, int b, int c) = (10, 20, 30);

// A tuple can combine several types, as long as the types of both sides match:
(x, bool f, double d) = (5, true, 3.14);

// Unlike other languages, the assignment is processed item-by-item:
(x, y) = (5, 10);
(x, y) = (y, x);
// now a and b are both 10! Swapping is not possible.

// When you embed the statement it returns the first item's value:
if ( (f, a) = (true, x) ) {
// always executed
}
Note that tuples only exist as a helper construct for assignments. You can not use
operators on them, they are not represented by an object, they can not be
used like arrays etc.



Now that there are tuples it becomes easy to extend the function syntax to have
several return values - just return a tuple:

// implementation:
(int, bool) parseInt(String str) {
char s[] = str.ToCharArray();
int r = 0;
foreach (char c in s) {
if ((c < '0') || (c > '9'))
return (0, false);
r = r * 10 + (c - '0');
}
return (r, true);
}

// invocation:
(int v, bool s) = parseInt("2004");
What I like most about that syntax is that it makes the code more compact. In
this example it removed 3 lines-of-code. It is also a nice solution for optional return values.

If you don't need the second return value, just write

int v = parseInt("2004");
You can name the return value and then use it like a C# reference
argument. The C# function

void inc2(ref int number1, ref int number2) {
number1++;
number2++;
}
could be written as

(int number1, int number2) inc2(int number1, int number2) {
number1++;
number2++;
}
Note that input and output values have the same name and no return
statement is needed, since the return values are named and already set.
When you name output variables you can also combine initialized return
values with the return statement. Here's an alternative implementation for
parseInt():

(int r = 0, bool success = true) parseInt(String str) {
char s[] = str.ToCharArray();
foreach (char c in s) {
if ((c < '0') || (c > '9'))
return (0, false);
r = r * 10 + (c - '0');
}
}
Another two LOC's less. As tuples can have only a single item, it's also possible
to use the same syntax for a function with only one return value:

(int r) add(int a, int b) {
r = a+b;
}
To summarize it, I think that tuples are a better solution for the multiple-return-value problem than argument references. They feel more natural because they bundle input and output values in declaration and invocation. The concept is closer to the Smalltalk concepts of messages, which makes it easier to create bindings to message-based protocols like SOAP. And last but not least it would help you to write shorter code.




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